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Effects of Framework Disruption of Ga and Ba Containing Zeolitic catalysts Article Effects of Framework Disruption of Ga and Ba Containing Zeolitic Materials by Thermal Treatment Siyabonga S. Ndlela 1 , Holger B. Friedrich 1,* and Mduduzi N. Cele 2 1 School of Chemistry and Physics, Westville Campus, University of KwaZulu–Natal, Private Bag X 54001, Durban 4000, South Africa; [email protected] 2 School of Physical and Chemical Sciences, Mafikeng Campus, North-West University, Private Bag X 2046, Mmabatho 2745, South Africa; [email protected] * Correspondence: [email protected]; Tel.: +27-31-2603-107 Received: 21 July 2020; Accepted: 29 July 2020; Published: 30 August 2020 Abstract: The effect of the thermal treatment of some zeolitic materials was studied on oxidative dehydrogenation (ODH) of n-octane. Gallium containing faujasite catalysts were synthesized using isomorphic substitution, specifically, a galosilicalite (Ga-BaY(Sil)) and an aluminosilicalite substituted with gallium (Ga-BaY(IS)), with constant Si/M ratio. The catalysts were thermally treated at different temperatures (250, 550, and 750 ◦C) before catalytic testing. The quantification of total and strength of acid sites by FT-IR (O-H region), pyridine-IR, and NH3-temperature-programmed desorption (TPD) confirmed a decrease in the number of Brønsted acid sites and an increase in the number of Lewis acid sites upon increasing the calcination temperature. Isothermal n-octane conversion also decreased with the catalysts’ calcination temperature, whereas octene selectivity showed the opposite trend (also at iso-conversion). The COx selectivity showed a decrease over the catalysts calcined from 250 to 550 ◦C and then an increase over the 750 ◦C calcined catalysts, which was due to the strong adsorption of products to strong Lewis acid sites on the catalysts leading to the deep oxidation of the products. Only olefinic-cracked products were observed over the 750 ◦C calcined catalysts. This suggested that the thermal treatment increases Lewis acid sites, which activate n-octane using a bimolecular mechanism, instead of a monomolecular mechanism. Keywords: oxidative dehydrogenation; n-octane; faujasite; zeolite; silicalite; isomorphic substitution 1. Introduction Zeolite materials offer a variety of properties in catalysis, resulting from their ability to be easily tuned both during synthesis and post synthesis. The introduction of other multivalent metals such as Fe, Ga, V, and Ti in the framework of zeolites can be carried out by substitution of aluminum during synthesis by an isomorphic substitution method [1,2]. Other cations, such as charge balancing cations on the extra framework of the zeolite (Na, K, Ba, etc.) can be introduced after synthesis by ionic exchange and impregnation [2]. Zeolites, however, are not ideal catalysts in the oxidative dehydrogenation of alkanes because of their highly acidic nature. The acidity of zeolites leads to unwanted side reactions that are selective to cracked products, catalyst deactivation as a result of coke deposition, and high COx production [3,4]. Therefore, it is of necessity to modify the acidic properties of zeolites to benefit the oxidative dehydrogenation (ODH) reactions in order to exploit other good properties (high surface area, thermal stability, high crystallinity, well-defined pores) of zeolites [1,2]. The introduction of group two alkaline earth metals by ionic exchange has shown to decrease the strength and overall acidity of the zeolite [5]. The introduction of other multivalent metals in the framework of a zeolite other than aluminum has also shown to decrease the acidity of zeolites [6]. Catalysts 2020, 10, 975; doi:10.3390/catal10090975 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 975 2 of 13 These modifications of zeolites have led to the synthesis of aluminum-free materials called silicalites, (Scheme S1) which have less acidity compared to the unmodified aluminosilicalite (zeolite). Zeolites and their analogues contain both Brønsted acid sites and Lewis acid sites, and these sites play different roles in the ODH of alkanes. Depending on the concentration of each acid site, the zeolite can either be a Brønsted acid or Lewis acid zeolite. Brønsted acid sites are associated with the hydroxyl groups bridging between the Si(OH)T groups of the zeolitic material, (T = multivalent metal), whereas Lewis acid sites are associated with the metallic species of the framework and extra framework of the material [7]. For hydrocarbon reactions, the Brønsted acid sites are known to directly activate the hydrocarbon by protonation of the feed molecule, whereas Lewis acid sites are known to induce a strong electrostatic field that can polarize adsorbed molecules which results in their activation [8]. Zeolitic materials treated at temperatures below 450 ◦C are mostly Brønsted acidic and prevalently associated with the Si(OH)T sites. Thermally treating the zeolitic materials causes the T atoms to migrate into partial or total extra framework positions (Scheme S2). This is true for samples treated at temperatures around 650 ◦C and for samples with the lowest Si/M ratios, such as faujasites [9]. Partial migration of T atoms means that the framework has been disrupted, but the atoms are still linked to the framework in a form of low coordinated and isolated species. These species at lattice defects are of strong Lewis character. The final step of this migration ends with the complete breaking of the bonds linking the T atoms to the framework, leading to the free migration of T atoms to Lewis acid centers and the formation of a silanol nest [9]. In ODH reactions, it is important, therefore, to understand the migration of the zeolitic material’s acid sites, as this influences the cracking mechanism, as well as the overall feed molecule activation mechanism. It is known that in alkane cracking on zeolitic materials, there is an involvement of carbonium ions as well as carbenium ions which leads to cracking via a ß-scission. The formation of the two species leads to two mechanisms; namely, monomolecular and bimolecular mechanisms for Brønsted acid sites and Lewis acid sites, respectively [10,11]. The monomolecular mechanism is the more pronounced and faster of the two mechanisms, it proceeds via the feed molecule protonation, resulting in a pentacoordinated carbonium ion which can crack to give off a smaller alkane fragment and an adsorbed carbenium ion. This ion subsequently cracks by ß-scission resulting in an olefin and a smaller carbenium ion. Lewis acid sites facilitated bimolecular mechanisms proceed via a hydride ion abstraction by a Lewis acid site from the feed molecule to form a carbenium ion which then cracks via a repeated ß-scission [10,11]. Therefore, by tracking the reaction product selectivity and conversion, one can probe the dominating mechanism in the ODH of n-octane when using thermally modified zeolitic materials. n-Octane was chosen as the representative medium to long chain alkane, which are very abundant and of low value. The ODH reaction was studied because it offers an interesting and less energy-intensive route for the production of olefins from paraffins by rendering the reaction exothermic as a result of the insertion of an oxygen source into the reaction, thus suppressing the thermodynamic constraints [12]. 2. Results and Discussion 2.1. Catalysts Characterization 2.1.1. Powder XRD The XRD pattern of both the prepared catalysts (Figure S1) showed the already known crystal structure of the faujusite type zeolite with characteristic reflections centered around 21.8◦, 26.4◦, and 54.5◦ [13]. There were no diffraction peaks observed for any gallium oxide species which suggests good dispersion of the metal within the zeolite framework. Complete substitution of aluminum with gallium in the zeolite framework did not affect the crystal structure of the zeolite, as both the aluminosilicalite and galosilicalite catalysts showed similar diffractograms. A slight shifting of peaks was observed which could be attributed to the difference in the size of aluminum and gallium in the zeolitic framework. Catalysts 2020, 10, 975 3 of 13 Catalysts 2020, 10, x FOR PEER REVIEW 3 of 13 was2.1.2. observed FT-IR which could be attributed to the difference in the size of aluminum and gallium in the zeolitic framework. The IR assignments for the two prepared catalysts are similar to previously reported faujasite 2.1.2.type FT-IR zeolitic materials (Figure S2) [12]. The shifts of the IR bands observed between the two spectra obtained can be attributed to change in lattice parameters induced by the substitution of aluminum The IR assignments for the two prepared catalysts are similar to previously1 reported faujasite typewith zeolitic gallium. materials The IR(Figure spectra S2) showed[12]. The bandsshifts of just the aboveIR bands 1800 observed cm− which between are the assigned two spectra to asymmetric 1 obtainedstretching can ofbe theattributed internal to tetrahedra,change in lattice and pa symmetricrameters induced stretching by the was substitution shown by of bands aluminum at 740–880 cm− . withThese gallium. bands The are IR weaker spectra for showed Ga-BaY(IS), bands just and above that could1800 cm be−1 attributed which are toassigned the di fftoerence asymmetric in crystal growth stretchingin the presence of the internal of aluminum tetrahedra, compared and symme to thetric stretching aluminum-free was shown Ga-BaY(Sil) by bands [ 14at ].740–880 The first cm− band1. around 1 These420 cmbands− is are indicative weaker for of theGa-BaY(IS), T-O bend, and and that the co externaluld be attributed linkage doubleto the difference ring was in shown crystal by the bands 1 1 growtharound in the 600 presence cm− . There of aluminum is a decrease compared in intensityto the aluminum-free of a ~3600 Ga-BaY(Sil) cm− band [14]. for The Ga-BaY(Sil), first band which is a aroundband 420 associated cm−1 is indicative with the of hydroxyl the T-O bend, group and attached the external to extra linkage framework double ring aluminum was shown [15 by– 17the].
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